Plastid quinones

Tyrosine monooxygenase, 4-hydroxy-phenylpyruvate dioxygenase, homogentisate prenylase (D 22, D 22.4) Further enzymes of plastid quinone biosynthesis Plastid quinones,... 

Excess amounts of plastid quinones and carotenoids are accumulated in plastoglobuli which are found in chloroplasts as well as in chromoplasts. During synthesis of certain types of chromo-plasts tubular structures are formed from thylakoids which contain appreciably amounts of carotenoids. In other types myelin-like carotenoid-containing membrane convolutes are built After massive accumulation of single carotenoid species large membrane-covered crystals, e.g., of j5-carotene and lycopene may be detected. 

Typical examples are experiments in which COg was administered to intact leaves in competition with mevalonic acid in the location of the site of plastid quinone biosynthesis and experiments with Penicillium cyclopium, where the channeling of the precursor L-phenylalanine and its incorporation into the alkaloids of the cyclopenin-viridicatin group and proteins were studied... 

Like chlorophyll, plastoquinone A has a nonpolar terpenoid or isoprenoid tail, which can stabilize the molecule at the proper location in the lamellar membranes of chloroplasts via hydrophobic reactions with other membrane components. When donating or accepting electrons, plastoquinones have characteristic absorption changes in the UV near 250 to 260, 290, and 320 nm that can be monitored to study their electron transfer reactions. (Plastoquinone refers to a quinone found in a plastid such as a chloroplast these quinones have various numbers of isoprenoid residues, such as nine for plastoquinone A, the most common plastoquinone in higher plants see above.) The plastoquinones involved in photosynthetic electron transport are divided into two categories (1) the two plastoquinones that rapidly receive single electrons from Peso (Qa and Qb) and (2) a mobile group or pool of about 10 plastoquinones that subsequently receives two electrons (plus two H+ s) from QB (all of these quinones occur in the lamellar membranes see Table 5-3). From the plastoquinone pool, electrons move to the cytochrome b f complex. 

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